Synthetic biology is limited by the small number of available and well-characterized transcription factors from which to program large genetic circuits. Bacterial sigma factors (as), the promoter recognition subunits of RNA polymerase (RNAP), are modular proteins with domains that recognize DNA sequences in the −10 and −35 regions of their target promoters1. In addition to the housekeeping σs (e.g., σ70 in E. coli) that recognize the thousands of canonical promoters essential for growth, bacteria have a variable number of stress-activated alternative σs that direct RNAP to distinct promoter sequences. This enables cells to express multiple genes associated with a particular developmental state or stress response2 and execute complex gene expression dynamics that implement temporal control and serve as developmental checkpoints3. For example, spore formation in B. subtilis requires a cascade of 5 σs (σH→σF→σE→σG→σK)4. σs can be embedded in complex webs of partner swapping networks, including anti-σs, which physically block σs from interacting with RNAP5-7, and anti-anti-σs. Such feedback loops and protein-protein interactions generate more complex dynamics for integrating many environmental and cellular signals8.